1. Field of the Invention
The invention relates to a device for supplying current to a filament of a cathode of an X-ray tube and for regulating said current at a selected value.
An X-ray tube is generally constituted like a diode, i.e. it is constituted by two electrodes, one of which, called cathode, emits electrons while the other, called anode, receives these electrons on a small surface that constitutes the source of X-radiation.
The cathode has a heated filament that constitutes the electron source. When the high voltage, provided by a generator, is applied to the terminals of both electrodes so that the cathode is at a negative potential, a so-called anode current is set up through the generator and crosses the space between the cathode and the anode in the form of an electron beam. The intensity of this electron beam depends on the temperature of the filament, this temperature being a function of the power dissipated in the filament, i.e. a function of the current, called the heating current, that flows in the filament.
The quantity of X-rays emitted by the anode depends mainly on the intensity of the anode current and, hence, on the heating current of the filament. Thus, the heating current of the filament is one of the major parameters that have to be determined for each radiographic or radioscopic exposure during a radiological examination of a patient.
The parameters of the exposure are determined as a function of the nature of the examination. As a rule, these parameters are pre-determined by an operator who displays their values on a control panel used to control the operation of the different elements of a radiological installation, for example the high-voltage generator and the filament current generator. Increasingly, the values of these parameters are being determined by means of a microprocessor device that computes the optimum values of these parameters, for example, according to the type of examination desired by the practitioner and the specific features of the installation.
The parameters that are computed are, for example, the duration of the exposure time, the energy of the X-radiation through the choice of the value of the high voltage applied between the cathode and the anode and the intensity of the anode current through the choice, notably, of a value of the intensity of the heating current of the filament.
It must be noted that the heating current should be capable of being substantially modified between one exposure and the next one, for example from 1.5 amperes to 5.5 amperes, as well as during the exposure time. Furthermore, these current values should be obtained quickly and automatically, and should be maintained for the requisite time.
2. Description of the Prior Art
There are many devices for supplying and regulating the current used to heat a filament of an X-ray tube cathode. One of these devices is described in the French patent No. 2 597 285. This prior art device, the schematic diagram of which is given in FIG. 1, includes a current supply circuit 11 and a regulation circuit 12 (including circuit 8).
The supply circuit 11 comprises a DC voltage source 13 represented by a battery cell 13' and a DC/AC converter 14. The DC/AC converter 14 includes a chopper circuit 31 comprising two switches 20 and 21 controlled by a control circuit 19 and diodes 22 and 23, and a resonant circuit 10 comprising capacitors 24 and 25 and a coil 26. The resonant circuit 10 is connected to a primary winding 28 of an isolating transformer 9, the secondary circuit 27 of which includes the filament 15 of the cathode of the X-ray tube 30. The filament 15 is, if necessary, supplied by means of a rectifier circuit 29 which, for example, is of the type comprising rectifying diodes and filtering capacitors.
The regulation circuit 12 has a circuit 8 for detecting the heating current of the filament and a circuit 16 for the measurement of this heating current of the filament, a comparator circuit 17 for comparing the measured current with a predetermined value, known as a set-point value Ic, a voltage-frequency converter circuit 18, the output signals of which are applied to the DC/AC converter 14 so as to modify its frequency and thus obtain a heating current, the value of which is equal to the set-point value Ic.
The device briefly described hereabove has the following drawbacks. The quick turning-off of the transistors of the switches 20 and 21 of the DC/AC converter 14 gives rise to quick variations in the current. These quick variations create parasitic signals which disturb the surrounding circuits, notably the primary winding 28 of the transformer which includes the heating current detection circuit 8. The measurement signals thus include parasitic elements which create an error in the regulation circuit 12. When this error is reproducible, it may be corrected by calibration of the device.
Such reproducibility is possible only if the operating state is stable. This is obtained by a regulation of the voltage E of the source 13.
The quick turning-off of the transistors of the DC/AC converter 14 also has the effect that the current that flows in the filament 15 has a form that changes between the sinusoidal form and the sawtooth form when the voltage E of the source varies. Now, in the regulation circuit 12, it is provided for a computation of the effective current that characterises the temperature of the filament by squaring the measurement value. When the heating current approaches the sawtooth form, its squared value has peaks corresponding to high-order harmonics that the effective value measurement circuit cannot reproduce because its passband is insufficient. A known way of avoiding such a phenomenon is to regulate the voltage E of the voltage source 13.